Time Zones and Tide Streams
with Poole Sailing
Time Zones
The world is divided, roughly along meridians of longitude, into different time zones. Because Greenwich meridian, zero
longitude runs through the British Isles we are in the fortunate position of being in time zone Z, or Zulu. It used to be
called Greenwich mean time but is now known as Co-ordinated Universal Time or UTC.
The Tide Table extract from the Almanac for Portsmouth
below shows that Portsmouth is time zone UT, the UK’s
Standard Time. The times on the page are all UT times. They
will equate to the time on our watches in winter but from
March 26th onwards we will have to add one hour to the
times shown so that the they equate to to the UK’s daylight
saving time called British Summer Time or BST.
The Almanac extract for Cherbourg shows that France is in time
zone UT-1. That means that you must subtract 1 hour from the
times shown on the page to turn them into UT times. So French
Standard time is one hour ahead of the UK standard time. France
also changes to Daylight Savings time on March 26th therefore,
French Summer time is still one hour ahead of British Summer
time but two hours ahead of UT.
Time Zones
It is easy to think that the time
zones don’t matter much but take
a look at this example.
We are leaving the UK on June 3rd
heading for St. Vaast in Normandy.
We know that the Gates to the
harbour at St. Vaast are tide
dependent and we have a table
reserved at a restaurant for nine
pm. From the Tide Table opposite
we can see that High Water at
Cherbourg will be at 19.28 UT-1 on
Friday June 3rd. We need to look at
the detail for St. Vaast in the
almanac.
Time Zones
We can see from the almanac that given a Cherbourg
HW at 19.28, the local High Water at St. Vaast will be
56 minutes later at 20.24. But that is in Time Zone
UT-1, so we must add one hour to turn that into local,
French Summer Time. Therefore HW St. Vaast will be
at 21.24 French Summer time. The almanac also says
that the Harbour Gate will open at HW – 2hrs
15mins. and then close at HW + 3hrs 30mins. So we
can expect the gate to open at 19.09 and close at
00.54.
Of course we are approaching the harbour with our
watches set to British Summer Time, which is 1 hour
behind French Summer Time. So our watches will say
18.09 when the gate opens and 23.54 when it closes.
We can enter the Harbour then, as long as we arrive
between 18.09 BST and 23.54 BST but we need to
remember that our table reservation is at 21.00 local
time, so we need to be at the restaurant when our
watch says 20.00 BST or we may be disappointed.
Tide Streams
•
•
•
•
•
•
•
We need to establish the speed and
direction of tide streams in order to
navigate in tidal waters or to work out an
efficient passage plan.
You will find ‘Tidal Diamonds’, like diamond
‘B’ circled at the top of the chart, scattered
around our charts.
Tidal diamonds are locations for tide
stream information, speed and direction of
the current.
Speed and direction is tabulated, bottom
right corner of the chart, for each hour of
the tidal cycle.
The information has to be referenced to a
port, in this case St. Helier, in Jersey.
We are given the direction of the current
as a true bearing plus the speed of the
current at the average spring tide and at
the average neap tide.
The chart shows that the tide stream at
location B will have a direction of 090
degrees true with a speed of 3.4 knots at
Springs or 1.6 knots at neaps, when it is
High Water at St. Helier.
Tide Streams
•
•
•
•
•
•
•
Most navigators prefer to use a
pictorial representation of the tide
information.
Opposite is a page from a Tidal Stream
Atlas for the Channel Islands. The atlas
comprises a single A4 page for each
hour of the tidal cycle.
The page shown shows the situation
when it is High Water at St. Helier,
Jersey.
Direction of the tide stream is shown
by the arrows, the bolder the arrow
the stronger the stream.
Strength of the stream is represented
by two numbers. The larger number is
the speed of the tide at the average
spring tide while the smaller number is
the speed at the average neap tide.
Both numbers are tenths of knots.
The current six miles north of
Cherbourg is running due east(090
degrees true) 1.3 knots at neaps or 3.2
knots at springs.
To put real time to the information we
need to know the time of High Water
at St Helier.
Tide Streams
•
•
•
•
•
•
•
•
The tide table opposite is covered in
information.
It tells us that St. Helier is in in time zone UT
and, therefore, that all of the times on the
page are in UT, the British standard time.
We can use them immediately in winter but
will need to add one hour in summer to turn
the times into BST, British Summer Time.
We are given the times and heights of high
and low waters for each day of the year.
The dates of the neap tides are shown in
blue and the dates of the spring tides in red.
They align closely with the timing of new
and full moons, also shown.
The difference between the height of high
and low water on each tide cycle is the
Range of Tide.
The range of tide is the amount that the tide
will rise or fall in the given cycle and has a
direct relationship to the speed of the tide
stream.
•
•
•
•
•
Tide Streams
To determine an accurate tide stream speed when we are neither at Springs or Neaps, we can use the computation
of rates table to be found on the inside cover of our tide stream atlas.
From the location on the appropriate page of the Tide Stream Atlas, take the average spring and the average neap
rates.
Mark the average spring rate on the dotted ‘spring’ line, and the average neap rate on the dotted ‘neap’ line. Then
draw a diagonal line through the two marks.
From the almanac find the range of tide for that tide cycle and mark the value on the vertical ‘range’ scale.
Now follow a horizontal line from the range to the diagonal line and then go up, or down, vertically to the value on
the Rate scale. You now have the Speed of the stream appropriate to that particular Range.